Back in the day ~23 years ago. A telco that had just launched started providing voice mail service. Normal outgoing call rates were about .5 usd per minute. The newly launched voice mail service was free. So university students found a new trick to call their friends, flash your friend (single ring signal to say that they don’t pick up their phone), hang up and call again, let it ring till it goes to voicemail, leave a long message, hang up and wait for the reply. Obviously it’s not full duplex and latency was high but what the e heck it’s free. The telco killed it within 2 days… and then we started having fun with all those smsc Center numbers from all over the world…
